Thyroid hormone receptors are being studied in rat liver and GH3 pituitary cell nuclei. The interaction of the hormone receptors with the nucleohistone will be the main focus of our continuing work. Nucleohistone will be prepared by shearing chromatin by mechanical and enzymatic techniques. The necleohistones will then be analyzed by sucrose gradient centrifugation. The relationship between hormone binding to receptors and biologic response will also be studied in detail.